The epidermis, the outermost layer of the skin, comprises a cellular continuum of four layers: the stratum corneum, the granular layer, the spinous layer, and the basal layer. Each cellular layer in the epidermis represents various stages along a process in which basal epidermal keratinocytes undergo a continuous cycle of proliferation, differentiation, and apoptosis, moving upward from the basal layer to finally yield corneocytes. These corneocytes form the cornified layer known as the stratum corneum.
Basal keratinocytes reside at the lower portion of the epidermis. These mitotically active cells undergo a proliferative cycle to generate daughter cells that are physically dislocated upward into the spinous and granular layers and undergo the process of differentiation into corneocytes. On passing through the spinous and granular layers, the cells undergo morphological changes that render them flatter in structure as they lose their cellular viability, undergo alternate keratin expression profiles, and transform into cellular remnants On average, a younger-aged epidermis turns over in about one month, shedding the older cells and replacing them with newer ones, but this process can increase to over forty days in older skin.
The stratum corneum's corneocytes remain connected to one other via proteins and lipids, creating a protective barrier between the organism and its outside environment. This tightly regulated epidermal permeability barrier functions as a physical and selective barrier against chemical and biological insults. Important functions of this barrier include attenuation of the penetration of free radicals and prevention of penetration of harmful radiation, including UV radiation, into deeper layers. The stratum corneum also acts as a permeability barrier and functions to prevent loss of body moisture to the outside environment. Dysfunction of this barrier can lead to chronic skin conditions, diseases, and in extreme cases can even threaten the viability of the organism.
Skin aging is a multifactorial process driven by both intrinsic (chronological aging) and extrinsic (environmental) factors, including ultraviolet (UV) exposure, environmental toxins, pollutants, and smoking. It is well known in the art that the ability of the stratum corneum to cyclically generate new layers of skin diminishes with age so that the stratum corneum turnover rate is substantially reduced in aged skin, with the cornified layer becoming gradually thinner. This results in a reduction in the functioning capacity of the barrier so that harmful stimuli penetrate the stratum corneum more easily, leading to UV-damage, for example, of the underlying dermal layers, degradation of collagen and elastin, and eventually manifests in appearance as wrinkling and skin atrophy. Thinning of the stratum corneum by the sum of intrinsic and extrinsic aging factors increases the visible appearance of fine lines and wrinkles. Further, the barrier suffers from an age-related increase in permeability to free radicals and a reduction in the amount of lipid in the intercellular matrix, decreasing barrier capacity to diffuse toxins from deeper layers. Recovery capacity of the barrier to environmental insult is also substantially reduced with age.
Thus, the skin's epidermal barrier function is key to the skin's ability to regenerate and protect itself from the appearance of aging signs such as fine lines and wrinkles. Accordingly, it would be desirable to provide compositions and methods of treatment that can improve the skin's epidermal functioning and thus also improve the appearance of aging skin.